Organic electroluminescent (EL) devices or organic light-emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. Tang et al. in Applied Physics Letters 51, p 913, 1987; Journal of Applied Physics, 65, p3610, 1989; and commonly assigned U.S. Pat. No. 4,769,292 demonstrated highly efficient OLEDs. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Organic light-emitting devices (OLED) generally can have two formats known as small molecule devices such as disclosed in commonly-assigned U.S. Pat. No. 4,476,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic light-emitting element, and a cathode. The organic light-emitting element disposed between the anode and the cathode commonly includes an organic hole-transporting layer an light-emitting layer and an organic electron-transporting layer. Holes and electrons recombine and emit light in the EL layer. Tang et al. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and commonly assigned U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron-transporting layer and the hole-transporting layer and recombine in the light-emitting layer. Many factors determine the efficiency of this light generating process. It has been found, however, that one of the key factors that limits the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as the bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent electrode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as the top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light can actually emit from the device and perform useful functions.
Madigan et al (Appl. Phys. Lett, Vol 76, No. 13, p 1650, 2000) taught the use of high index substrates with micro-lens to enhance the light extraction efficiency. Matterson et al (Adv. Mater. 2001, 13, No. 2, 2001), Lupton et al (Appl. Phys. Lett. Vol 77, No. 21, p3340, 2000) taught the use of corrugated substrates to improve light extraction. Gu et al (Optics Letters, Vol. 22, No. 6, p. 396, 1997) taught the use of substrates with special shaped micro-structures to improve light extraction. Gifford et al (Appl. Phys. Lett. Vol. 80, No. 20, p. 3679, 2002) taught the use of substrates with periodical structure and opaque metal layer to enhance light coupling through surface plasmon cross coupling. All these methods, however, suffer the common problem of much increased complexity in the device construction and at the same time produce light outputs that have high angular and wavelength dependence which are not suitable for many practical applications.
Another common problem frequently encounter in fabrication of OLED device is the yield problem. Because of the small separation between the anode and the cathode, the OLED devices are prone to shorting defects. Pinholes, cracks, steps in the structure of OLED devices, and roughness of the coatings, etc. can cause direct contacts between the anode and the cathode or to cause the organic layer thickness to be smaller in these defective areas. These defective areas provide low resistance pathways for the current to flow causing less or, in the extreme cases, no current to flow through the organic EL element. The luminous output of the OLED devices is thereby reduced or eradicated. In a multi-pixel display device, the shorting defects could result in dead pixels that do not emit light or emit below average intensity of light causing reduced display quality. In lighting or other low resolution applications, the shorting defects could result in a significant fraction of area non-functional. Because of the concerns on shorting defects, the fabrication of OLED devices is typically done in clean rooms. But even a clean environment cannot be effective in eliminating the shorting defects. In many cases the thickness of the organic layers is also increased to beyond what is actually needed for functioning devices in order to increase the separation between the two electrodes to reduce the number of shorting defects. These approaches add costs to OLED device manufacturing, and even with these approaches the shorting defects cannot be totally eliminated.
JP2002100483A discloses a method to reduce the shorting defect due to local protrusions of crystalline transparent conductive films of an anode by depositing an amorphous transparent conductive film over the crystalline transparent conductive film. It alleged that the smooth surface of the amorphous film could prevent the local protrusions from the crystalline films from forming shorting defects or dark spots in the OLED device. The effectiveness of the method is doubtful since the vacuum deposition process used to produce the amorphous transparent conductive films does not have leveling functions and the surface of the amorphous transparent conductive films is expected to replicate that of the underlying crystalline transparent conductive films. Furthermore, the method does not address the pinhole problems due to dust particles, flakes, structural discontinuities, or other causes that are prevalent in OLED manufacturing processes.
JP2002208479A discloses a method to reduce shorting defects by laminating an intermediate resistor film made of a transparent metal oxide of which, the film thickness is 10 nm-10 μm, the resistance in the direction of film thickness is 0.01-2 Ω-cm2, and the ionization energy at the surface of the resistor film is 5.1 eV or more, on the whole or partial of light emission area on a positive electrode or a negative electrode formed into transparent electrode pattern which is formed on a transparent substrate made of glass or resin. Although the method has its merits, the specified resistivity range cannot effectively reduce leakage due to shorting in many OLED displays or devices. Furthermore, the ionization energy requirement severely limits the choice of materials and it does not guarantee appropriate hole injection that is known to be critical to achieving good performance and lifetime in OLED devices. Furthermore, the high ionization energy materials cannot provide electron injection and therefore cannot be applied between the cathode and the organic light emitting layers. It is often desirable to apply the resistive film between the cathode material and the organic light emitting layers or to apply the resistive film both between the cathode and the organic light emitting materials and between the anode and the organic light emitting materials.